Brain Soup

By Shlomo Maital

Suzana Herculano-Houzel

How do you count the number of neurons (brain cells) in a brain (whether human or animal)?   Dr. Suzana Herculano-Houzel, Vanderbilt University, has found a creative way.

   Previously, the method was to take samples of brain tissue, freeze it, put it under a microscope and count neurons. But this was inaccurate, because neuron density in brains varies widely, depending on the place within the brain.   Using this method we thought the brain had 100 billion neurons. That’s not a lot – elephants have three times more!

   Herculano-Houzel read an old 1970’s study, suggesting, why not measure the amount of DNA in a brain, then divide by the amount of DNA per neuron?   Hmmm… problem is, DNA per neuron varies widely.

   So she developed a new, creative method.   Take a brain.   Puree it using a blender. (Honest!).   Brain soup, she calls it.   Mark the neurons with a chemical dye, then mark again with a red dye to mark the nucleus of the neurons. Neurons have only one nucleus, like all cells. So if you count the neuron nucleuses, you can compute how many neurons there are in the brain.

   Answer?   86 billion.   Or, 14% fewer than we thought (100 b.).

   So why are humans so smart? The key part of the brain, that makes us smart, is the cerebral cortex, that wrinkled outer part of the brain. Because it is wrinkled, it has a lot of surface area, enabling more neurons to pack it.   Turns out we have 16 b. neurons in the cerebral cortex, while orangutans and gorillas have 9 billion, and chimps have 6 billion. (Those are respectable numbers – those primates are clever!).

     And those 16 b. neurons in the cerebral cortex are waht makes us smart, and it is probably where Dr. Suzana got the idea…   Hey, why did no-one else think of it before?

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